The development of mobile telecommunications continues towards ever smaller and increasingly complicated handheld units. The development has recently lead to new requirements for handheld units, namely that the units should support several different standards and telecommunications systems. Supporting several different systems requires several sets of filters and other RF components in the RF parts of the handheld units. Despite this complexity, the size of a handheld unit should not increase as a result of such a wide support.
The RF filters used in prior art mobile phones are usually discrete surface acoustic wave (SAW) or ceramic filters. This approach has been adequate for single standard phones, but does not allow support of several telecommunications systems without increasing the size of a mobile phone.
Surface acoustic wave (SAW) resonators typically have a structure similar to that shown in FIG. 1. Surface acoustic resonators utilize surface acoustic vibration modes of a solid surface, in which modes the vibration is confined to the surface of the solid, decaying quickly away from the surface. A SAW resonator typically comprises a piezoelectric layer 100, and two electrodes 122, 124. Various resonator structures such as filters are produced with SAW resonators. A SAW resonator has the advantage of having a very small size, but unfortunately cannot withstand high power levels.
It is known to construct thin film bulk acoustic wave resonators on semiconductor wafers, such as silicon (Si) or gallium arsenide (GaAs) wafers. For example, in an article entitled "Acoustic Bulk Wave Composite Resonators", Applied Physics Letters, Vol. 38, No. 3, pp. 125-127, Feb. 1, 1981, by K. M. Lakin and J. S. Wang, an acoustic bulk wave resonator is disclosed which comprises a thin film piezoelectric layers of zinc oxide (ZnO) sputtered over a thin membrane of silicon (Si). Further, in an article entitled "An Air-Gap Type Piezoelectric Composite Thin Film Resonator", I5 Proc. 39th Annual Symp. Freq. Control, pp. 361-366, 1985, by Hiroaki Satoh, Yasuo Ebata, Hitoshi Suzuki, and Choji Narahara, a bulk acoustic wave resonator having a bridge structure is disclosed.
FIG. 2 shows one example of a bulk acoustic wave resonator having a bridge structure. The structure comprises a membrane 130 deposited on a substrate 200. The resonator further comprises a bottom electrode 110 on the membrane, a piezoelectric layer 100, and a top electrode 120. A gap 210 is created between the membrane and the substrate by etching away a sacrificial layer. The gap serves as an acoustic isolator, essentially isolating the vibrating resonator structure from the substrate.
Bulk acoustic wave resonators are not yet in widespread use, partly due to the reason that feasible ways of combining such resonators with other circuitry have not been presented. However, BAW resonators have some advantages as compared to SAW resonators. For example, BAW structures have a better tolerance of high power levels.
Micromechanical devices are also presently under development. A micromechanical device is created typically on silicon substrates using deposition, patterning and etching techniques to create the desired structure. As an example, FIG. 3 illustrates the structure of a micromechanical switch. A micromechanical switch comprises a cantilever 400, contact pads 430 on the substrate 200 and a contacting bar 440 for creating a contact between the contact pads 430, and two electrodes 410, 420. The cantilever electrode 410 is formed on the cantilever and the substrate electrode 420 on the substrate. The contacting bar is formed at one end of the cantilever, and the other end of the cantilever is fixed to the substrate, preferably with a support 405 in order to raise the cantilever away from the substrate surface. The micromechanical switch is operated with a DC voltage coupled between the cantilever and substrate electrodes. The DC voltage creates an electrostatic force between the cantilever and substrate electrodes of the switch. The electrostatic force bends the cantilever, bringing the contacting bar into contact with the substrate contact pads 430. Various other micromechanical structures are disclosed in an article entitled "Ferroelectric Thin Films in Microelectromechanical Systems Applications", MRS Bulletin, July 1996, pp. 59-65, by D. L. Polla and L. F. Francis, and references contained therein.